The invention relates to a method of monitoring tap changers.
The Japanese Patent Publication Sho-60-176213, laid open on Sep. 10, 1985, describes a tap changer with a motor drive for actuating same. The motor drive and the driven tap changer are thus connected with one another by a drive shaft train. For monitoring the orderly function of the tap changer, with each selection of the tap, the instantaneous torque on the drive shaft is ascertained and is compared, as an actual value, with a setpoint value which is determined by a respective previously determined switch-specific torque. Upon deviations which exceed a certain threshold, a defect alert is generated.
The Japanese Patent Publication Sho-62-172240, laid open on Nov. 11, 1987, also relates to such a monitoring method based upon which a monitoring of the torque at the drive shaft is carried out and describes a torque detection unit which is arranged directly on the drive shaft, which measures the torque in a direct mechanical manner and which converts the torque into an electrical value and thereafter compares the electrical value with corresponding setpoint.
From the presentation of Professor Gorgius xe2x80x9cTechnical Diagnostics on Tap Changers For Large Power Transformersxe2x80x9d at the xe2x80x9c8th International IMEKO Symposium on Technical Diagnosticsxe2x80x9d and the Symposium Papers published in relation thereto, such a method is also already known in which the mechanical torque at the drive shaft is determined in an indirect manner by measurement of the power which is required for the drive motor. By known measuring devices, current and voltage converters, current converters, voltage and cos xcfx86 at the drive motor are measured and the power Pw is calculated. From the respective specific motor characteristics Md=f(Pw), the torque can be calculated.
This indirect determination of the torque of the drive shaft provides an elegant and relatively simple solution. On the one hand, it avoids the need for torque measuring hubs or other mechanical measuring means on the drive shaft, and on the other hand, it eliminates the otherwise required conversion of the measured mechanical parameter into an electrical parameter better suited for the subsequent actual-value/setpoint comparison. In spite of these advantages, monitoring of tap changes by a torque comparison, based upon the power demand of the drive motor has up to now not been practiced. The main reason for this is that a calculator-determined torque more accurately represents the torque of the drive motor or, stated more accurately the torque at the end of the drive shaft which is connected to the drive train for operating the tap changer. What should be monitored, therefore, is not the torque at this location but more properly the torque at the other end of this drive train, namely, at the tap changer.
In FIG. 1 this relationship has been shown schematically: In a transformer tank T, a tap changer OLTC is recessed. Laterally of the transformer tank T, a motor drive MA is disposed, from which a drive shaft train W1, W2, W3 runs to the tap changer OLTC. With the process described above the torque MPw is obtained at the location 1 at which is found the drive motor for the motor drive. Instead, the relevant torque is a torque MSt arising at the location 2 which is within the tap changer. What is the difficulty is that there is no constant mathematical relationship between the calculated torque MPw and the real torque in the tap changer MSt. Rather the relationship varies as a result of temperature effects, aging and wear characteristics of the mechanism of the tap changer, the bearing locations and other effects during the whole life of the tap changer which as a rule can extend over many years.
There is thus no stable, i.e. constant, transfer relationship between the drive motor of the motor drive and the tap changer driven based upon the time-course of the torque.
The object of the invention is, therefore, to provide a method for the purposes described which effects with each switching a calibration, i.e. a new calculation of the relationship between MPw and MSt and thus of the transfer function which allows the determination of the instantaneous real torque independently from the mentioned influencing factors to enable an effective monitoring and to provide a more effective setpoint/actual value comparison.
This object is achieved with a method of monitoring a tap changer whereby during the actuation of the tap changer, i.e. during the switchover process, current, voltage and phase angle of an electric drive motor of a motor drive provided for actuating the tap changer are substantially continuously measured. From these measured values the respective actual power draw of the drive motor is calculated. From the calculated power draw and based upon the specific motor characteristic the effective torque associated therewith is calculated.
With each switching process (x=1,2,3, . . . ) of the tap changer from the magnitudes of the calculated values for the torque at the drive motor (MxPw1 . . . MPxP8) the characteristic values at two previously established time points (tA, tB) which are the same for each switchover process are selected for the torque at the drive motor (MxPwA, MxPwB) and stored.
From the magnitudes of the calculated values for the torque at the drive motor (MxPw1 . . . MxPw8) the corresponding values for the torque at the tap changer (MxSt1 . . . MxSt8) are calculated in accordance with the relationship
MxSt1=MxPw1xc2x7mx+nx, . . . ,MxSt8=MxPw8xc2x7mx+nx,
and these calculated values are monitored and compared with previously established threshold values, whereby the values for mx and nx are calculated for the torque at the drive motor (Mxxe2x88x921PwA,Mxxe2x88x921PwB) on the one hand and at the tap changer (Mxxe2x88x921StA, Mxxe2x88x921StB) on the other hand each at the respective one of the two time points (tA,tB) according to the relationships       M    x    =                              M                      x            -            1                          ⁢        StB            -                        M                      x            -            1                          ⁢        StA                                      M                      x            -            1                          ⁢        PwB            -                        M                      x            -            1                          ⁢        PwA            
From the magnitudes of these calculated values for the torque at the tap changer (MxSt1 . . . Mx St8) the characteristic values at the two previously established time points (tA, tB) for the torque at the tap changer (Mx StA, Mx StB) are also selected and stored. After each switchover process, the characteristic values for the previous switching (xxe2x88x921) for the torque at the drive motor (Mxxe2x88x921PwA,Mxxe2x88x921PwB) and at the tap changer (Mxxe2x88x921StA,Mxxe2x88x921StB) are overwritten by the corresponding values of the immediately concluded subsequent switching (x). Prior to the first time switch operation (x=1), i.e. in the unactuated new state of the tap switch or after it has been reset, the corresponding values for m1 and n1 are predetermined from an external source. For calculation of mx an additional memory factor xcex can be considered which is less than 1 and from which a corrected value mx is obtained in accordance with the relationship mxe2x80x2x=xcexxc2x7mxxe2x88x921+(1xe2x88x92xcex)xc2x7mx.
Each calculation of mx is compared with the value mxxe2x88x921 obtained and stored in the preceding switch operation and when mx less than 0 or mx greater than mxxe2x88x921+a, where a is predetermined constant, mx=mxxe2x88x921 is valid, i.e. no change for this value is effective over the previous last switching (xxe2x88x921). The values for the torque at the motor (MxPw1 . . . MxPW8) and/or at the tap changer (MxSt1 . . . MxSt8) is subdivided into typical functional regions or time regions, so-called windows, and for each window depending upon a previously developed calculation a typical single value for the corresponding torque is calculated such that the typical values of all windows provide a basis for monitoring the further process.
The advantages of the method according to the invention are that the evaluation of each already completed switching or a plurality of previously effected switching of the tap changer results in an actual matching of the transfer stretch, i.e. of the relationship MSt=f(Mpw). More accurately stated: for the calculation of the torque upon an actual switching of the tap changer, corresponding parameters are obtained from the previous (last) switching to determine the transfer function, etc.
The method of the invention can be made finer however. It is especially advantageous when, in addition, a so-called memory factor is introduced. This memory factor which is incorporated in the respective transfer function and is newly calculated upon the actual switching of the tap changer, utilizes the value for a preceding switching only in part, i.e. the respective actual slope in the transfer function is calculated to provide a newly calculated slope and combined with the slope from the previous switching. This has the considerable advantage that values which are imprecise are utilized only to a reduced extent since they are only involved in part in the calculation step. In an especially advantageous embodiment of the method, plausibility tests are carried out and the effect of an error linked calculation of the increase can thereby be reduced.